Synchronous dynamic random access memory

ABSTRACT

A synchronous dynamic random-access memory has transparent latch circuits that latch address signals in synchronization with a clock signal. An X-address is latched following activation of a first control signal; a Y-address is latched following activation of a second control signal. Data selected by the latched X- and Y-addresses are held in a data latch and output through a tri-state output circuit in synchronization with the clock signal. Data output starts a certain number of clock cycles from activation of the first control signal. The data can be output for one clock cycle, the same data can be output for two or more consecutive clock cycles, or different data can be output in consecutive clock cycles in an order determined by certain bits in the X- and Y-addresses.

This is a Division of allowed application Ser. No. 08/144,557 filed Nov.20, 1993, now U.S. Pat. No. 5,339,276 which is a divisional of priorallowed application Ser. No. 07/791,388 filed Nov. 14, 1991, now U.S.Pat. No. 5,287,327.

BACKGROUND OF THE INVENTION

This invention relates to a dynamic random-access memory (DRAM) thatoperates in synchronization with a clock signal, thus being suitable forintegration into microcontrollers and application-specific integratedcircuits.

A microcontroller, for example, generally comprises a central processingunit, read-only memory, random-access memory, and other modulesintegrated onto a single semiconductor chip. Operations of all on-chipcircuits are synchronized by a system clock signal. In the past, therandom-access memory has generally been of the static type (SRAM), whichhas a simple control scheme permitting rapid access and can easily besynchronized to the system clock. For many purposes it would bedesirable to use DRAM, Which takes up less space and dissipates lesspower than SRAM, but the more complex control required by DRAM has beenan obstacle.

In prior-art DRAM, the timing of address input and data output isdetermined by a plurality of control signals. Even if these controlsignals are generated by the central processing unit in synchronizationwith the system clock, because of unavoidable fabrication variations incircuit elements, synchronization between the system clock and DRAMinput and output operations will be inaccurate. Inaccuratesynchronization must be allowed for by employing a low clock frequency,which of course limits the speed of the entire device.

SUMMARY OF THE INVENTION

It is accordingly an object of the present invention to synchronize DRAMaddress input accurately to a clock signal.

Another object of the invention is to synchronize DRAM data outputaccurately to a clock signal.

Yet another object of the invention is to output data in a single clockcycle selected in relation to a first control signal.

Still another object is to enable data output to be extended foradditional clock cycles responsive to a second control signal.

A further object is to enable data from different memory cells to beoutput in consecutive clock cycles.

A still further object is to enable the order of data output inconsecutive clock cycles to be altered according to certain addressbits.

A synchronous dynamic random-access memory has address input means forlatching an address signal on a first transition of a clock signal togenerate an X-address, and on a second transition of the clock signal togenerate a Y-address. The first transition is selected by a firstcontrol signal; the second transition is selected by a second controlsignal. A decoding means selects one or more memory cells in a memorycell array according to the X-address and Y-address. An output meansoutputs data from the selected memory cells in synchronization with theclock signal.

According to one aspect of the invention, the output means comprises adata latch means, tri-state output means, and output control means. Dataread from the memory cell array are held in the data latch means, andoutput by the tri-state output means in response to an output controlsignal from the output control means. The output control means activatesthe output control signal for one or more clock cycles, starting acertain number of clock cycles after activation of the first controlsignal.

According to another aspect of the invention, the output means comprisesa multiple data latch means, a tri-state output means, and a consecutiveoutput control means. The multiple data latch means latches and holdsdata read from a plurality of memory cells in the memory cell array. Theconsecutive output control means receives one or more address bits fromthe address input means, selects the data held in the multiple datalatch means in an order determined from these one or more address bits,and supplies the selected data to the tri-state output means forexternal output.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a synchronous DRAM.

FIG. 2 is a schematic diagram of a novel address input circuit.

FIG. 3 is a timing diagram for FIG. 2.

FIG. 4 is a schematic diagram of a modification of the address inputcircuit of FIG. 2.

FIG. 5 is a timing diagram for FIG. 4.

FIG. 6 is a block diagram of a synchronous DRAM with novel output means.

FIG. 7 is a schematic diagram illustrating one configuration of theoutput control circuit in FIG. 6.

FIG. 8 is a timing diagram for FIG. 6 and FIG. 7.

FIG. 9 is a schematic diagram illustrating another configuration of theoutput control circuit in FIG. 6.

FIG. 10 is a timing diagram for FIG. 6 and FIG. 9.

FIG. 11 is a block diagram of a synchronous DRAM with another noveloutput means.

FIG. 12 is a timing diagram for FIG. 11.

FIG. 18 is a block diagram of a synchronous DRAM with yet another noveloutput means.

FIG. 14 is a timing diagram for FIG. 13.

FIG. 15 is a timing diagram for FIG. 13.

DETAILED DESCRIPTION OF THE INVENTION

A more detailed description of the invention will be given withreference to the drawings. These drawings are shown as illustrations ofthe invention but they do not restrict its scope as set forth in theappended claims.

Referring to FIG. 1, the invented memory comprises an address inputcircuit 10 that receives n+1 address signals A0-An (n is a positiveinteger), a clock signal (CLK), a first control signal (conventionallydesignated RAS: row address strobe), and a second control signal (CAS:column address strobe). Address input is multiplexed: both a row address(X-address) and column address (Y-address) are input on the same signallines A0-An. The address input circuit latches the address signals on afirst transition of CLK selected by RAS, the latched address becoming anX-address (AX0-AXn), then latches the address signals again on a secondtransition of CLK selected by CAS, the address latched this timebecoming a Y-address (AY0-AYn).

The X-address is supplied to an X-address decoder 20 and the Y-addressto a Y-address decoder 30. These address decoding means decode theX-address and Y-address to select one or more memory cells in a memorycell array 40. A data input/output means 50 transfers data between theselected memory cell or cells and an external data bus (not shown)thereby either reading data out of the memory cells or writing data intothe memory cells. Data output is synchronized with CLK.

FIG. 2 is a schematic diagram of the address input circuit 10 in FIG. 1.The address signals A0-An are supplied to a first transparent latchcircuit 17 and a second transparent latch circuit 18. The address inputcircuit also has a third transparent latch circuit 11 for latching RAS,and a fourth transparent latch circuit 12 for latching CAS.

A transparent latch circuit is controlled by an enable signal, beingtransparent to its input when the enable signal is in a first state,latching its input when the enable signal changes to a second state, andcontinuing to output the latched value until the enable signal revertsto the first state. In FIG. 2 the first transparent latch circuit 17 iscontrolled by an X-address latch signal S13, the second transparentlatch circuit 18 is controlled by a Y-address latch signal S14, and thethird and fourth transparent latch circuits 11 and 12 are controlled byCLK. Specifically, the first transparent latch circuit 17 is enabled ormade transparent when the X-address latch signal S13 is low, and latchesthe address signals A0-An upon low-to-high transition of the X-addresslatch signal S13. The second transparent latch circuit 18 is madetransparent when the Y-address latch signal S14 is low, and latches theaddress signals A0-An via an input gate 16 upon low-to-high transitionof the Y-address latch signal S14. The third and fourth transparentlatch circuits 11 and 12 are made transparent when CLK is Low and latchthe RAS or CAS upon, respectively, low-to-high transition of CLK.

The X-address latch signal S13 is output by an X-address latch signalgenerating circuit 13 comprising an inverter 13a that inverts RAS, aNAND gate 13b that performs the NOT AND logic operation on the output ofthe inverter 13a and CLK, and a set-reset flip-flop 13e comprising NANDgates 13c and 13d. The set-reset flip-flop 13e is set by the output ofthe NAND gate 13b and reset by the output of the inverter 13a; ittherefore sets when RAS is low and CLK is high, and resets when RAS goeshigh. The X-address latch signal S13 is the output of the NAND gate 13c.

The operation of the X-address latch signal generating circuit 13 isevident from the schematic. S13 is inactive (low) whenever RAS isinactive (high). If RAS becomes active (low) while CLK is low, at firstthere is no change in S13, but as soon as CLK goes high, the set-resetflip-flop 13e sets and S13 becomes active (high). Thereafter, S13remains active (high) until RAS becomes inactive (high) again.

The Y-address latch signal S14 is output by a Y-address latch signalgenerating circuit 14 similar in structure and operation to theX-address latch signal generating circuit 13, having an inverter 14a, aNAND gate 14b, and a set-reset flip-flop circuit 14e comprising NANDgates 14c, and 14d. The Y-address latch signal S14 is inactive (low)whenever CAS is inactive (high), becomes active (high) when CAS isactive (low) and CLK is high, and remains active until CAS becomesinactive (high) again.

To avoid unwanted changes in the output of the second transparent latch18, this address input circuit also has a delay line 15 and a Y-addressinput gate 16. The delay line 15 comprises, for example, a D-typeflip-flop that delays the output of the first transparent latch circuit11 by a certain amount responsive to CLK, also inverting the output ofthe third transparent latch 11, thus producing a signal S15 that becomesactive (high) at a certain transition of CLK after RAS has become active(low). The Y-address input gate 16 performs an AND logic operation onS15 and each of the address bits A0-An, thus feeding the address signalsA0-An to the second transparent latch circuit 18 when S15 is active andblocking the address signal A0-An when S15 is inactive.

The operation of FIG. 2 will be explained with reference to FIG. 3,which is a timing diagram for FIG. 2. Cross-hatching In FIG. 3 indicatesthe don't-care state.

RAS is driven active (low) by an external device such as a centralprocessing unit at a time t1 after the fall of CLK, allowing a certainsetup time Trs with respect to the next low-to-high transition of CLK.At about the same time, the central processing unit outputs an X-addresson the address signal lines A0-An. As soon as it becomes valid, thisaddress is passed through the first transparent latch circuit 17 to theX-address decoder 20 as the X-address AX0-AXn. Since the Y-address inputgate 16 is closed, this address is not passed to the second transparentlatch 18 or the Y-address decoder 30.

At time t2 CLK goes high, the X-address latch signal S13 rises, and thefirst transparent latch circuit 17 closes. The first transparent latchcircuit 17 thus continues to output the X-address AX0-AXn even after theaddress signal A0-An changes. At the next high-to-low transition of CLKthe signal S15 goes high, opening the Y-address input gate 16.

At time t3, following this high-to-low transition of CLK and allowing acertain setup time Tcs with respect to the next low-to-high transitionof CLK, the central processing unit drives CAS active (low). At aboutthe same time the central processing unit places a Y-address on theaddress signal lines A0-An. As soon as it becomes valid, this address ispassed through the Y-address input gate 16 and the second transparentlatch circuit 18 to the Y-address decoder 30 as the Y-address AY0-AYn.

At time t4 CLK goes high, the Y-address latch signal S14 rises, and thesecond transparent latch circuit 18 closes, thus continuing to outputthe Y-address AY0-AYn even after the address signals A0-An changes.

The address signals A0-An are synchronized with CLK, so by latching thesignals at the CLK transitions at t2 and t4 it is easy to ensure thatvalid addresses are latched. In addition, since RAS and CAS are not usedas latch triggers (as in the prior art), but only to determine thetriggering edges of CLK, their timing requirements are relaxed. RAS andCAS can be driven active even prior to valid address output, as in FIG.3. High clock rates are thus possible.

Moreover, the use of transparent latch circuits 17 and 18 allowsdecoding of the X- and Y-addresses to begin even before the latch pointsat t2 and t4, resulting in an overall shortening of the memory cycle.Similar advantages apply to the latching of RAS and CAS by thetransparent latch circuits 11 and 12, the outputs of which are furnishedto other circuits such as data input and output timing circuits notshown in FIG. 2.

The address input circuit 10 in FIG. 1 is not limited to theconfiguration shown in FIG. 2. For example, the third and fourthtransparent latches 11 and 12, the delay line 15, and the Y-addressinput gate 16 can be omitted if not needed. The internal structure ofthe X- and Y-address latch signal generating circuits 13 and 14 can bemodified. First and second control signals that are active high can beused instead of RAS and CAS. X- and Y-address buffer circuits may beadded as output stages following the first and second transparent latchcircuits 17 and 18.

A modification of the address input circuit of FIG. 2 is shown in FIG.4. The address input circuit of FIG. 4 differs from the address inputcircuit of FIG. 2 in that the input of the X- and Y-address latch signalgenerating circuits 13 and 14 are connected respectively to the outputsof the third and fourth transparent latch circuits 11 and 12. With thismodification, the timing requirements of RAS and CAS further relaxed, aswill be seen from FIG. 5. That is, if the RAS goes low at t1' beforehigh-to-low transition of CLK, the X-address latch signal S13 does notgo high until t2 because the output S11 of the third transparent latchcircuit 11 goes low at or slightly after (with the propagation delay)the high-to-low transition of CLK. The time at which the X-address islatched is therefore unaffected. Similarly, even if the CAS goes low att3' before high-to-low transition of CLK, the Y-address latch signal S14does not go high until t4 because the output S12 of the fourthtransparent latch circuit 12 goes low at or slightly after (with thepropagation delay) the high-to-low transition of CLK. The time at whichthe Y-address is latched is therefore unaffected.

The remainder of the detailed description of the invention is concernedwith the output section of the data input/output circuit 50 in FIG. 1,and will show several novel means of outputting memory data insynchronization with CLK.

FIG. 6 is a block diagram showing one basic data output scheme. Elementscommon to FIG. 1 have the same reference numerals. The address inputcircuit 10 can be the circuit shown in FIG. 2 or a modification thereofas noted above.

Output data from the memory cell array 40 is amplified by senseamplifiers (not explicitly shown) and placed on an internal data bus 51.After further amplification by an amplifier circuit 60, the data is sentto a data latch circuit 70 and held for output. Output takes placethrough a tri-state output circuit 80 capable of producing high or lowoutput, or placing the output signal line or lines (Do in the drawing)in a high-impedance state. The output circuit is enabled and disabled byan output control signal S90 generated by an output control circuit 90.

Also shown in FIG. 6 is a memory control signal generating circuit 100comprising a latch circuit 101 and a signal generating circuit 102. Thelatch circuit 101 receives CLK and external control signals such as RAS,CAS, an output enable signal (OE), and a write enable signal (WE), andhas the general function of latching the control signals andsynchronizing the operation of the DRAM with CLK. The latch circuit 101comprises, for example, circuits similar to those in FIG. 2. (The fourthtransparent latch 12 in FIG. 2, which is not explicitly used for addressinput, can be considered as disposed in the latch circuit 101.) Thesignal generating circuit 102 receives the outputs of the latch circuit101 and generates various types of memory control signals, such as asignal selecting read or write mode, which are not explicitly shown butwill be familiar to those skilled in the art.

The output control circuit 90 is adapted to drive the output controlsignal S90 to the active state for one clock cycle, starting a certainnumber of clocks after RAS becomes active: for example, S90 may beactive during an interval from substantially the third rising edge ofCLK after RAS becomes active until substantially the fourth rising edge.In order for S90 to be activated. CAS must become active after RASbecomes active, but it is not necessary for CAS to remain active. Theoutput control circuit 90 can be realized in various ways, one of whichis described next.

Referring to FIG. 7, the output control circuit 90 comprises an inverter91 for inverting RAS, a set-reset flip-flop 92-1 that is set by CAS andreset by the output of the inverter 91, and D-type flip-flops 92-2 to92-5 connected in series and clocked by CLK. The output of the set-resetflip-flop 92-1 is fed to a further D-type flip-flop 93-1, which is alsoclocked by CLK.

The first flip-flop 92-2 in the series of D-type flip-flops 92-2 to 92-5receives the inverted RAS signal from the inverter 91, and passes thissignal to the second flip-flop 92-3. From the second flip-flop 92-3, thesignal enters the next-to-last flip-flop 92-4. The output 992-4 of thenext-to-last flip-flop 92-4 is fed not only to the last flip-flop 92-5but also to an AND gate 93-2 comprising a NAND gate followed by aninverter. The other input of the AND gate 93-2 is the output of thefurther flip-flop 93-1.

The output of the AND gate 93-2 is fed to a one-shot pulse generatingcircuit 94-1 comprising an odd number of inverters and a NAND gateconnected in series. The output of the last flip-flop 92-5 is fed to asimilar one-shot pulse generating circuit 94-2. The outputs of theone-shot pulse generating circuits 94-1 and 94-2 are fed as set andreset inputs, respectively, to a set-reset flip-flop 92-6. The output ofthe set-reset flip-flop 92-6 is the output control signal S90.

The purpose of the set-reset flip-flop 92-1, the further flip-flop 93-1,and the AND gate 93-2 is to ensure that the output control signal S90does not go high unless CAS becomes active (low) beforehand.

The operation of the memory In FIG. 6 and FIG. 7 will be explained withreference to FIG. 8. Address input is carried out as already described:an X-address is latched at the first rising edge of CLK after RASbecomes active, at time t11 in FIG. 8; a Y-address Is latched at thefirst rising edge of CLK after CAS becomes active, at time t12 in FIG.8. Decoding of the X- and Y-addresses begins even before t11 and t12, sodata output from the memory cell array 40 starts promptly around timet12, permitting the data to be latched in the data latch circuit 70before the next rising CLK edge at time t13.

The lower eight waveforms in FIG. 8 illustrate the operation of theoutput control circuit 90 and tri-state output circuit 80. When CAS goesactive (low) between times t11 and t12, the set-reset flip-flop 92-1sets. The signal S92-1 goes high at this time and remains highregardless of the further behavior of CAS. The flip-flop 93-1 latchesthe signal S92-1 at time t12, causing the signal S93-1 to go high atsubstantially time t12.

The fall of RAS before time t11 causes a high signal to be latched inthe flip-flop 92-2 at time t11, in the flip-flop 92-3 at time t12, andin the flip-flop 92-4 at time t13. The signal S92-4 thus goes highsubstantially at time t13. Since S93-1 is already high, the output ofthe AND gate 93-2 changes from low to high, causing the one-shot pulsegenerating circuit 94-1 to output a brief low pulse S94-1 that sets theset-reset flip-flop 92-6. The output control signal S90 thus becomesactive (high) at substantially time t13.

At time t14 the signal S92-4 is latched in the flip-flop 92-5, theoutput of which thus changes from low to high, causing the one-shotpulse generating circuit 94-2 to produce a brief low pulse S94-2 thatresets the set-reset flip-flop 92-6. The output control signal S90therefore becomes inactive (low) at substantially time t14.

The tri-state output circuit 80 is normally in the high-impedance state,but when the output control signal S90 is active, the tri-state outputcircuit 80 is enabled and outputs data Do. Data output thus takes placefor one CLK cycle from substantially time t13 to substantially time t14,regardless of the active duration of CAS. The data can be read at orbefore time t14.

Although not shown in the drawing, at an appropriate time RAS goes highto end the memory cycle. RAS can be driven high at any time after timet13, and the low-to-high transition of RAS need not be preciselysynchronized with the time at which the output data are read.

An advantage of the memory illustrated in FIG. 6 to FIG. 8 is that dataoutput is accurately synchronized to CLK regardless of the precisetiming of RAS and CAS. The same is true of the latching of addressinput. Operation of the memory can thus easily be synchronized with theoperation of other circuits disposed, for example, on the samesemiconductor chip and driven by the same clock signal CLK.

FIG. 9 illustrates another possible configuration of the output controlcircuit 90. Elements common to FIG. 7 have the same reference numerals,except that the output of the set-reset flip-flop 92-6 is denoted S90A.

FIG. 9 is identical to FIG. 7 except for the addition of an OR gate 93-3and an output extending circuit 95. The output extending circuit 95receives CLK, CAS, S92-4, and the output of the inverter 91 and producesan output extending signal S95. The OR gate 93-3, which comprises a NORgate followed by an Inverter, combines the signal S90A with the signalS95 by OR logic to produce the output control signal S90 sent to thetri-state output circuit 80 in FIG. 6.

The output extending circuit 95 comprises a logic circuit 95a, aset-reset flip-flop 95b, and an inverter 95c. The logic circuit 95acomprises logic gates and a flip-flop (FF) that is triggered on risingedges of CLK. It can readily be verified from FIG. 9 that the set-resetflip-flop 95b resets, driving the signal S95 high, when CAS and RAS areboth low and S92-4 is high, and sets, driving the signal S95 low, whenCAS is high and CLK goes high, or when RAS is high, or when S92-4 islow.

The operation of the output control circuit in FIG. 9 will be explainedwith reference to the timing diagram in FIG. 10, which illustrates twocases: a first case (a) in which CAS goes high before the output cyclebegins at time t13, and a second case (b) in which CAS goes high aftertime t13.

In case (a), the signal S95 output by the output extending circuit 95remains low throughout the memory cycle, because S92-4 never becomeshigh while CAS is low. The operation is thus the same as in FIG. 8: dataDo are output for one CLK cycle from substantially time t13 tosubstantially time t14.

In case (b), CAS is still active (low) when S92-4 goes high at time t13,so the signal S95 becomes active (high) at this time. S95 remains activeuntil t15, the first rising edge of CLK after CAS becomes inactive(high). The high input of S95 to the OR gate 93-3 in FIG. 9 extends theactive (high) interval of the output control signal S90 until time t15.Data output thus continues for two CLK cycles from substantially timet13 to substantially time t15. The data can be read, for example, on thefirst rising edge of CLK after CAS becomes inactive.

The output control circuit in FIG. 9 provides the same advantages as theoutput control circuit in FIG. 7, output being accurately synchronizedto CLK regardless of the precise timing of CAS and RAS. A furtheradvantage is that by holding CAS low, the central processing unit canprolong data output until it becomes convenient to read the data.

FIG. 11 is a block diagram of a novel memory generally similar to theone in FIG. 6 but adapted for nibble-mode output. Elements common toFIG. 6 have the same reference numerals. Descriptions of the addressinput circuits will be omitted.

The nibble output mode is a mode in which input of a single pair of X-and Y-addresses causes output in four consecutive clock cycles of datafrom, for example, memory cells in four different quadrants of thememory cell array. If the width of the output data Do is one bit, thisoperation causes one nibble (four bits) to be output in four clockcycles. For simplicity, the subsequent description will be confined tothis case, but the invention is not limited to output of one bit at atime. The width of the output data Do may be eight bits, for example, inwhich case thirty-two bits are output in four clock cycles, eight bitsat a time. Neither is the invention limited to just four clock cycles;the same inventive concept can be used to obtain data output in anydesired number of consecutive clock cycles.

The memory in FIG. 11 has a data bus group 51A comprising four databuses, each similar to the data bus 51 in FIG. 6. These carry data fromseparate memory cells in the memory cell array 40 to a multiple latchcircuit 70A comprising, in this case, four one-bit latch circuits 70A-1to 70A-4. The outputs from the multiple latch circuit 70A are sent to aconsecutive output control circuit 110.

The consecutive output control circuit 110 comprises a decoder 111, ashift register 112, and an output select circuit 113. The decoder 111receives certain bits of the X-address AX0-AXn and the Y-address AY0-AYnfrom the address input circuit 10, such as the most significant addressbits AXn and AYn, decodes these bits, and sends four resulting signalsto the shift register 112.

For any input to the decoder 111, just one of the signals output fromthe decoder is active, the others being inactive. Provided thiscondition is met, the invention is not limited to any particulardecoding scheme. Nor is the invention limited to the use of the mostsignificant address bits; the decoder 111 can decode any address bit orset of address bits as required by the application.

The shift register 112 is well-known device that stores the outputs ofthe decoder 111 in four bit storage cells 112-1 to 112-4, and rotatesthe contents of these bit storage cells 112-1 to 112-4 insynchronization with the clock signal CLK. The rotation is from left toright, the contents of the last bit storage cell 112,4 being shiftedinto the first bit storage cell 112-1. The contents of the bit storagecells 112-1 to 112-4 are sent as outputs from the shift register 112 tothe output select circuit 113.

The output select circuit 113 comprises switches 113-1 to 113-4 thatturn on and off in response to the outputs of the shift register. Theseswitches connect the outputs of the one-bit latch circuits 70A-1 to70A-4 to the tri-state output circuit 80.

A circuit generally analogous to the output control circuit 90 in FIG.6, but with additional D-type flip-flops following the flip-flop 92-5,is incorporated into the signal-generating circuit 102 in FIG. 11. Usingthis circuit, the signal-generating circuit 102 generates a signal S90Bthat enables the tri-state output circuit 80 for four consecutive CLKcycles starting, for example, with the second cycle after RAS becomesactive.

The operation of the memory in FIG. 11 will be explained with referenceto FIG. 12. As before, the X-address is latched at the first rising edgeof CLK after the fall of RAS (time t21) and the Y-address is latched atthe first rising edge of CLK after the fall of CAS (time t22). Thedecoder 111 decodes the most significant address bits AXn and AYn. Attime t22 the four decoded signals from the decoder 111 are stored in theshift register 112. Also at time t22, four bits of data D1, D2, D3, andD4 are sent from the memory cell array 40 via the data bus group 51A tothe multiple data latch circuit 70A, and stored in the one-bit latchcircuits 70A-1 to 70A-4.

Since just one of the four decoded signals from the decoder 111 isactive, a logic "1" is stored in just one of the four bit storage cells112-1 to 112-4. Suppose, for example, that a logic "1" is stored in thebit storage cell 112-1 and a logic "0" in the bit storage cells 112-2 to112-4. Then the switch 113-1 turns on while the switches 113-2 to 113-4are off, and during the interval from substantially time t22 to time t23the bit D1 stored in the one-bit latch circuit 70A-1 is sent to thetri-state output circuit 80 and output as the output data Do.

When the clock signal CLK rises at time t23 the logic "1" is shiftedfrom the bit storage cell 112-1 to the bit storage cell 112-2, causingthe switch 113-1 to turn off and the switch 113-2 to turn on. During theinterval from substantially time t23 to time t24 the bit D2 stored inthe one-bit latch circuit 70A-2 is sent to the tri-state output circuit80 and output as the output data Do.

Similarly, at time t24 the logic "1" is shifted from the bit storagecell 112-2 to the bit storage cell 112-3 and the bit D3 stored in theone-bit latch circuit 70A-3 is output as the output data Do. At time t25the logic "1" is shifted from the bit storage cell 112-3 to the bitstorage cell 112-4 and the bit D4 stored in the one-bit latch circuit70A-4 is output as the output data Do.

An advantage of the memory described in FIG. 11 and FIG. 12 is thatnibble-mode output is obtained at the rate of one bit per clock cycle.In the prior art, nibble-mode output was obtained by toggling CAS. Iftransitions of CAS are synchronized with, say, the falling edge of CLK,then two CLK cycles are needed to produce one cycle of CAS, so data canbe output at only half the rate shown in FIG. 12.

FIG. 13 shows a modification of the memory in FIG. 11. Elements commonto FIG. 11 have the same reference numerals.

The new element in FIG. 13 is a shift control circuit 114 that receivesa shift control signal S111 from the decoder 111 and sends acorresponding signal to the shift register 112 to control the shiftdirection. When the shift control signal S111 is high, for example, theshift register 112 rotates from left to right, but when the shiftcontrol signal S111 is low the shift register 112 rotates from right toleft. The decoder generates S111 by decoding the most significantaddress bits AXn and AYn.

The operation of the memory in FIG. 13 will be described with referenceto FIG. 14 and FIG. 15.

Referring to FIG. 14, suppose that the most significant address bits AXnand AYn are decoded as in FIG. 12 so that the bit D1 stored in theone-bit latch circuit 70A-1 is output first, and suppose further thatfor this value of AXn and AYn, the shift control signal S111 is high.Then the output operation Is identical to FIG. 12, data being output inthe order D1, D2, D3, D4.

Referring to FIG. 15, suppose that the most significant address bits AXnand AYn have different values, causing the bit D2 stored in the one-bitlatch circuit 70A-2 to be output first and the shift control signal S111to be held low. Then-the order of output is D2, D1, D4, D3.

In some microcontroller designs it is convenient for the centralprocessing unit to be able to receive nibble data in either the forwardor reverse order, depending on the starting bit. The memory in FIG. 13can easily be interfaced to such a central processing unit. The centralprocessing unit can operate faster than In the prior art because it doesnot have to rearrange the data received from the memory.

The memories in FIG. 6. FIG. 11, and FIG. 13 also have circuits forwriting and refreshing data in the memory cell array 40. These circuitsare well known and for the sake of simplicity and clarity were omittedfrom the drawings.

The structures of the output control circuits in FIG. 7 and FIG. 9 andthe consecutive output control circuits in FIG. FIG. 11 and FIG. 13 canbe modified in various ways, and these circuits can be combined toprovide, for example, both single-bit and nibble output modes in thesame memory. These and other changes can be made without departing fromthe spirit and scope of the present invention, which should bedetermined solely from the appended claims.

What is claimed is:
 1. A synchronous dynamic random-access memory thatprovides data output in synchronization with a clock signal,comprising:a plurality of memory cells for storing data; an addresssignal output circuit for outputting address signals for selecting saidmemory cells, in synchronization with said clock signal; and a dataoutput circuit for temporarily storing data stored in ones of saidplurality of memory cells selected by said address signals, andconsecutively outputting, in synchronization with said clock signal, thedata thus temporarily stored.
 2. The synchronous dynamic random-accessmemory of claim 1, wherein said data output circuit comprises:a latchcircuit having a plurality of bits for temporarily storing data of saidmemory cells; and an output control circuit for consecutively readingthe data stored in the bits of said latch circuit.
 3. The synchronousdynamic random-access memory of claim 2, wherein said output controlcircuit comprises a shift register for output of signals thatconsecutively select the bits in said latch circuit.
 4. A method ofreading data from a synchronous dynamic random-access memory which has aplurality of memory cells for storing data and reads said data out at anoutput terminal in synchronization with a clock signal, comprising thesteps of:selecting data, in synchronization with said clock signal, in aplurality of said memory cells; temporarily storing the data thusselected; and selectively transferring one bit of the data thustemporarily stored to said output terminal, in synchronization with saidclock signal.
 5. The method of reading according to claim 4, wherein thestep of selectively transferring one bit of data thus temporarily storedcomprises the further steps of:transferring a first bit of data which,among said data thus temporarily stored, was stored in a memory cellwith a highest address; transferring a second bit of data which, amongsaid data thus temporarily stored, was stored in a memory cell with alowest address; and transferring a third bit of data which, among saiddata thus temporarily stored, was stored in a memory cell with anaddress intermediate between said highest address and said lowestaddress.
 6. The method of reading according to claim 5, wherein saidsecond bit is transferred after said first bit and before said thirdbit.
 7. The method of reading according to claim 5, wherein said thirdbit is transferred after said first bit and before said second bit.